Post-traumatic stress disorder (PTSD) is a relatively new addition to the clinical taxonomy of psychiatric disorders. It was included in the third edition of the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM-III) in 1980 on the basis of informed clinical opinion. As currently defined, the disorder is based on the general hypothesis that exposure to extreme stressors produces a specific syndrome of psychiatric symptoms. The purpose of this study is to examine that hypothesis empirically, using data from the recently-completed National Vietnam Veterans Readjustment Study. The study's specific aims are: (1) to examine the relationship between specific stressor characteristics and specific PTSD symptoms, (2) to examine the patterns of co-occurrence of PTSD symptoms and their relationship to stressor characteristics, (3) to examine in a multivariate framework the relationships between multiple stressor characteristics and the specific PTSD symptoms, while controlling for some potentially important individual background characteristics, and (4) to examine the relationship between stressor characteristics and other psychiatric symptoms not included in the official definition of the PTSD syndrome. The study will involve descriptive analyses of the relationship between specific PTSD symptoms and diagnostically-relevant stressor characteristics, and then multivariate analyses of the relationship between specific stressor characteristics and specific PTSD symptoms, PTSD symptom groups, and symptoms of other psychiatric disorders, taking account of selected individual background characteristics. This basic research on the phenomenology of PTSD and the interaction of personal background and event characteristics will provide information that will improve our understanding of the variability of psychological reactions to extreme stress, and ultimately lead to greater diagnostic specificity. The study is particularly timely in that the American Psychiatric Association is currently in the process of revising its diagnostic manual (to become DSM-IV), and the principal investigator is a member of the panel that is reviewing the diagnostic criteria for PTSD.